A computer may use hard-disk drives (HDD) for storing data that is needed for operation of the computer. A typical HDD includes a magnetic read/write head and one or more magnetic disks for storing data. But for clarity, hereinafter a HDD is discussed as including only a single disk. The disk is mounted on a spindle motor, which rotates the disk, thereby allowing the movable read/write head to read data from and to write data to the disk. The spindle motor and read/write head may be controlled by a power chipset.
When the HDD is powered down during a normal power-down sequence (e.g., when an operator properly shuts down the computer), the read/write head is typically moved to a safe location such that the head does not impact or come to rest on the disk. This operation of moving the head to a safe location is called parking the head. If the head is not parked before the HDD supply voltage falls below a certain level, then the head may land on the disk while or after the disk stops spinning, thus potentially damaging the disk and/or the read/write head.
When main power is suddenly lost in an HDD, the controlling power chipset may provide auxiliary power to the HDD for a limited amount of time. For example, the power chipset may convert the inertia of the still-spinning hard disk into electrical energy. This time and the auxiliary power provided by the chipset are typically sufficient for the HDD to safely park the read/write head.
In addition to parking the read/write head, an HDD may attempt to perform other tasks (e.g., write data to the disk) during this sudden power-down time. For example, one may wish to store system-restore information about an incomplete operation of the computer so that during or after the next power up of the computer, this information may be retrieved and the previously incomplete operation may continue. Therefore, as more and more power demands are placed on an HDD after power is lost, the danger increases that not enough auxiliary power will be available for parking the read/write head, and, therefore, that the hard disk itself may be damaged by the read/write head crashing into the disk.